Negative effective excitonic diffusion in monolayer transition metal dichalcogenides

Abstract

While exciton relaxation in transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton propagation has received only little attention -in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos).Here, we study the spatiotemporal dynamics in TMDs and track the way of optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative transient diffusion. This phenomenon can be traced back to the existence of dark exciton states in TMDs and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering.arXiv:1908.07735v1 [cond-mat.mes-hall]

Nonclassical Exciton Diffusion in Monolayer WSe2

“…As monolayers they host robust exciton states with high binding energies [16,17] and the possibility to carry spin-valley information quanta [18][19][20]. The excitons in TMDCs have been shown to be mobile [21][22][23][24][25][26][27][28], guided by gradients [29][30][31], exhibit nonlinear diffusion [32][33][34][35][36][37], strain dependence [38], as well as intriguing propagation in heterostructures [39][40][41][42].…”

“…This duality is expected to manifest itself prominently in the exciton transport behavior, including potential emergence of quantum interference phenomena [48][49][50][51][52]. Despite much progress, however, only little is known regarding the appropriate picture for the exciton propagation in monolayer semiconductors, currently based on the assumption of a purely semiclassical framework [22,24,28,30,32,33,36,37,53].…”

“…The required input parameters for monolayer WSe 2 are taken from first-principle studies [93,104]. The diffusion coefficient is extracted from the spatiotemporal evolution of the excitons in the semiclassical approximation neglecting exciton-exciton mechanisms [36][37][38]. The model takes explicitly into account both a realistic multivalley band structure of WSe 2 and the exciton-phonon coupling beyond the long-wavelength acoustic branches.…”

Phonon-assisted exciton dissociation in transition metal dichalcogenides

“…In this study, we discuss exciton dissociation in the four most studied semiconducting TMDs including WSe 2 , WS 2 , MoSe 2 , and MoS 2 . However, for now, we focus on hBN-encapsulated WSe 2 monolayers due to their well established excitonic landscape containing bright (KK) and momentum-dark (KΛ and KK') excitons [15,19,23,50,51]. We consider up to 7 bound s-states for each valley, assigning states with higher main quantum number to the continuum and disregarding states with non-zero angular momentum due to their lower exciton-phonon cross section [16].…”

“…The strong light-matter interaction and the ultrafast nonequilibrium dynamics [5][6][7][8][9] makes them excellent candidates for active materials in photodetectors and solar cells. The understanding of their properties has significantly advanced in the last years with experimental and theoretical studies shining light on the optical response [10][11][12][13][14], exciton relaxation dynamics [15][16][17][18][19] and exciton propagation [20][21][22][23][24] in different TMD materials. While these are important properties for the operation of a photodetector, exciton dissociation plays a central role as the bridge between optical excitation and photocurrent generation.…”